Stroboscope and Tachometer

We built our prototype using an Arduino Uno mainboard and a Jaycar XC4454 LCD Shield. Since the shield has pads to break out unused I/O pins, we simply soldered the required components to those pads on the shield. The LCD Shield also includes several tactile pushbuttons, so we have everything we need for a complete user interface.

The character (alphanumeric) LCD on the shield is driven in four-bit mode by pins D4-D7 of the Uno, with D8 and D9 providing the RS and E signals, respectively.

The pushbuttons are connected to a resistor chain that sends a different voltage to the A0 analog input, depending on which buttons are pressed.

Fig.1 shows how to wire up the external components. At the top, the white LED connects between D12 and D11 with a 220Ω resistor in series. This makes up the Stroboscope, with the processor driving D12 to control the flash rate. D11 is permanently held low to create a convenient alternative to a ground connection.

The IR LED is powered by the 5V and GND pins, so it is always on. Its job is to provide an IR light source for the IR photodiode to detect. With the arrangement we are using, the photodiode behaves somewhat like a solar cell, generating a voltage on its anode relative to the cathode.

Since the photodiode behaves more like a current source than a voltage source, a parallel resistor is provided to turn the current into a voltage that the ADC peripheral of the Uno can measure. We use a photodiode as they can respond faster than devices like LDRs.

For this project, we have used an Arduino Uno R3 as other processor boards like the Arduino Leonardo use their processor to handle their USB interface. Since the Uno has a separate USB interface chip, it has fewer interruptions, making it better at managing the precise timing needed in this project.

The software consists of an Arduino sketch and two libraries. The library to drive the LCD panel is included with the Arduino IDE, while an external ‘TimerOne’ library is used to manage the strobe timing.

The sketch sets up a timer interrupt to drive the white LED with a duty cycle of 10% (ie, off for nine times longer than it’s on) at a rate you can control. The strobe can also be switched off. Professional strobes use a much lower duty cycle at a higher power level to more accurately ‘freeze’ the view.

The sketch also samples the photodiode voltage at 10ms intervals (100 times per second), then calculates and displays a rate based on the time between detected pulses. The display can be set to RPM (revolutions per minute) or Hz (revolutions per second) for both the Strobe and Tachometer.

Start by soldering the LEDs to their 220Ω resistors by cutting each anode (longer) lead short. Cut down one lead of each 220Ω resistor to a similar length. Solder the resistor to the LED and use a few centimetres of heatshrink tubing to cover the resistor.

You can then solder the LED assemblies to the LCD shield as shown. The white LED connects between the second and third pads at the top of the shield, with the cathode on the third pad.

The IR LED (which is blue) is wired between 5V and GND, with its cathode to GND. Trim any excess lead length from these components.

Solder the 100kΩ resistor between the other GND pin and A1; it should be a comfortable fit. The longer anode lead of the photodiode is also soldered to A1, with the cathode going to GND. The active area of the photodiode is the curved lens, so bend its leads to point the lens in the same direction as the IR LED.

Finally, plug the LCD shield into the top of the Uno and hook it up to your computer for programming. You should see the power LED on the LCD shield light up.

You can download the Arduino sketch for this project (siliconchip.au/Shop/6/448). We have included a copy of the TimerOne library with the sketch download, but it can also be installed by searching for “timerone” in the Arduino Library Manager.

Use the Arduino IDE (download from www.arduino.cc/en/software) to upload the sketch to the Uno, being sure to select the correct port and use the Uno board profile. Screens 1, 2 & 3 show some of the typical displays.

To use it as a Stroboscope, shine the white LED at a rotating object and adjust the rate until the object appears stationary. Remember that the object will also appear stationary if the rate is a fraction (eg, 1/2 or 1/3) of the rotation speed. The correct rate is the highest rate at which the object appears stationary.

When using the Stroboscope, remember that the object that appears stationary might not be! This can be dangerous if that object is machinery, as you might be tempted to touch it. So take great care when using the Stroboscope near running machinery.

The Tachometer is used by aiming the IR LED and photodiode at a rotating object and reading out the value in the lower-right corner of the LCD screen. You should be able to get a reading of a few Hz or a few hundred RPM by waving your hand a few centimetres in front of the LED/photodiode.

If you don’t get a good reading, check that the IR LED is emitting by pointing a mobile phone camera at it. The camera should show a red or purple glow that isn’t visible to human eyes. Other IR emitting sources (eg, remote controls) might cause interference, so keep the unit away from them.

Remember that objects like fans with multiple blades will cause multiple events per revolution, so you may have to account for this in your calculations. One way around this is to place a piece of reflective tape on the object so that you can easily pick up one event per revolution.

The Stroboscope/Tachometer is a simple and handy tool for checking the speed of rotating objects. It may not be the best tool for calibrating heavy machinery, but we think it would be a convenient way to check if your record turntable is spinning at the correct rate, for example. To check a turntable rotation speed, you also need a separate strobe disc with markings.

This article appeared in the May 2024 issue of Silicon Chip Magazine. Check out their website for other project ideas: https://www.siliconchip.com.au/